An etching technique for forming a tappered metal layer is disclosed in PUPA 1-151236 (1989) which is more fully described in FIGS. 7-10 herein. Referring to FIGS. 7 through 10, cross-sectional diagrams are shown illustrating a method of the prior art for taper etching an aluminum layer in a sequence of steps.
As FIG. 7 shows, an aluminum layer 13 is formed on a substrate 11, and a molybdenum layer 14 is formed on the aluminum layer 13 using a thin film deposition method such as sputtering or evaporation. A photoresist 15 is applied on the molybdenum layer 14 to form a predetermined pattern. As FIG. 8 shows, when the substrate 11 is immersed in an etching solution consisting, for example, of phosphoric acid, nitric acid, and water, the molybdenum layer 14 is etched and the underlying aluminum layer 13 is exposed. On contacting the etching solution, the molybdenum layer 14 and the aluminum layer 13 are etched simultaneously, and the aluminum layer 13 is etched through the molybdenum layer 14 as a mask. At this time, since the molybdenum layer 14 has a higher etching rate than the aluminum layer 13, the cross-sectional pattern of the aluminum layer 13 becomes moderately tapered. Then, if the photoresist 15 is removed as shown in FIG. 9, the molybdenum layer 14 may be removed as FIG. 10 shows, molybdenum layer 14 may be etched off by dry etching to obtain the tapered pattern of the aluminum layer 13 on substrate 11.
Another etching technique of the prior art is disclosed in PUPA 61-77344 (1986), wherein the second thin film for example a W--Ti alloy film, Mo, other high-melting-point metals, or the alloys thereof is deposited on the first thin film of Al or of an Al--Si alloy film on which a tapered pattern is to be formed; a resist pattern is formed on the second thin film; and, when dry etching is performed using the second thin film as a mask, the edge of the second thin film is etched beyond the edge of the resist pattern. If dry etching is continued under different conditions, the second thin film functions as the second etching mask to form a tapered pattern on the sidewall of the first thin film.
In PUPA 1-95524 (1989), a method for forming a tapered cross-sectional pattern of a chromium film is disclosed. In this method, the first thin film of chromium is formed on a silicon substrate, the second thin film of aluminum or an aluminum alloy is formed on the first thin film, a desired resist pattern is formed on the second thin film, and the first and second thin films are etched using phosphoric acid to form a tapered cross-sectional pattern by the difference in etching rates of these layers.
In PUPA 3-263833 (1991), a method is disclosed in which a lower layer of gold is formed on an insulator substrate, an upper layer of gold having an etching rate higher than the lower layer is formed, a photoresist layer is formed on the upper layer, and the lower layer is tapered by wet etching.
All of the above prior art methods are directed to forming a tapered pattern on the sidewall of a lower layer having a lower etching rate than an upper layer by etching the lower and upper layers on a substrate. However, if the film growth temperature is high or the film growth time is long to form the layers, the diffusion length of metal ions composing the lower and upper layers increases making the diffusion of metals into each other occur easily, and an alloy is formed in the interface between the above metals.
Since an alloy has a different etching rate from the etching rate of the original metals, the alloy formed in the interface between the lower and upper layers may be etched faster, may be etched slower, or may be etched only a little. However, if the two layers and the alloy therebetween have different etching rates, taper etching, the object of this invention, cannot be achieved.
If an alloy layer having a lower etching rate is formed as granules in the interface between the lower and upper metal layers, the etchant permeates between the granules, removing the surrounding metal and forming vermicular defects 28 on metal wiring 23 on substrate 21 as shown in FIG. 11. These vermicular defects 28 increases the possibility of short-circuiting or open-circuiting due to the breaking of wires. Since the alloy has different sizes and forms depending on ambient factors such as temperature, the state of the interface between the two metal layers changes depending on the storage conditions of substrate 21, causing taper formation to be unstable.